Why does glaze bubble when fired

I am comparing 6 well known cone 6 fluid melt base glazes and have found some surprising things. The top row are 10 gram GBMF test balls of each melted down onto a tile to demonstrate melt fluidity and bubble populations.

Second, third, fourth rows show them on porcelain , buff, brown stonewares. The first column is a typical cone 6 boron-fluxed clear. The others add strontium, lithium and zinc or super-size the boron. They have more glassy smooth surfaces, less bubbles and would should give brilliant colors and reactive visual effects. The cost? They settle, crack, dust, gel, run during firing, craze or risk leaching. Out of this work came the GE and GF.

Cone 03 white stoneware with red terra cotta ball-milled slip and transparent overglaze. These are eye-popping stunning. They are test LU Ferro frit , 6 tile kaolin , Silica , near the final mix for a white low fire stoneware.

The GJ glaze is super clear. Two reasons. These were fired in a schedule designed to burn off the gases from the bentonite in the body before the glaze fuses it soaks the kiln for 2 hours at F. Terra cotta clays generate alot of gases at cone cone 03 producing glaze micro-bubbles , but here the terra cotta is only a thin slip over the much cleaner burning white body. GQ and J low fire ultra-clear glazes contain Ferro Frit , and clay fired across the range of to F these were 10 gram GBMF test balls that melted and flattened as they fired.

Notice how they soften over a wide range, starting below cone F! Of course, these specimens test a very thick layer, so the bubbles are expected. But they still can be an issue, even in a thin glaze layer on a piece of ware. So to get the most transparent possible result it is wise to fire tests to find the point where the glaze starts to soften in this case F , then soak the kiln just below that on the way up to fire away as much of the carbon as possible.

Of course, the glaze must have a low enough surface tension to release the bubbles, that is a separate issue. Both are zero porosity. The Polar Ice is very translucent , the P much less. The blue coloration of the P is mostly a product of the suspended micro-bubbles in the feldspar clear glaze GU. The cone 6 glaze is fritted and much more transparent, but it could be stained to match the blue.

These are high quality combinations of glaze and body. Although the recipe is well melted at cone 2, it is still not fluid enough to enable their migration in the time available. By contrast, the melt at the upper temperature is much less viscous, enabling all bubbles to completely clear on the thinner sections.

If this glaze were applied to ware it would be in a thin layer and the bubbles would likely clear at cone 6. Not to be ignored is the degree to which the thousands of bubbles passing upward through the melt have helped to mix the melt and remove discontinuities in the cone 7 and 8 specimens.

This is happening because this glaze lacks flux and is not fluid enough to enable their migration. In the upper half they are more evident double thickness.

The glaze on the far left? It is fired to cone 6. At lower temperatures carbonates and hydrates in body and glaze are more likely to form gas bubbles because that is where they are decomposing into the oxides that stay around and build the glass and the ones that are escaping as a gas. By cone 6 the bubbles have had lots of time to clear.

Notice the amount of bubbles due to the high LOI , or loss on ignition. The 3HX is flowing a little more. This could be because of a difference their proportions of dolomite and calcium carbonate minerals, the individual mineral purities or the particle size or all three.

Whatever the case, 3HX will make a glaze flow a slightly better. Two transparent glazes applied thickly and fired to cone 03 on a terra cotta body. Right: A commercial bottled clear, I had to paint it on in layers, I ended up getting it on pretty thick.

Left: GS, a mix of Ferro frits , nepheline syenite and kaolin - one dip for 2 seconds and it was glazed. And it went on more evenly.

Bubbles are of course generated by the body during firing. But also in the glaze. Low temperature glazes melt early, while gassing may still be happening.

Both of these could have been applied thinner. And I could have fired them using a drop-and-hold and a slow-cool schedule.

I melted these two 9 gram GBMF test balls on tiles to compare their melting the chemistry of these is identical, the recipes are different. That means the while the ulexite is decomposing during melting it is creating gases that are creating bubbles in the glass.

Notice the size of the F is greater because it is full of bubbles. While this seems like a serious problem, in practice the F fires crystal-clear on most ware. Medium temperature transparents do not shed micro bubbles as well, clouds of these can dull the underlying colors.

Cone 6 transparents must be applied thicker. The stains used to make the underglazes may be incompatible with the chemistry of the clear glaze less likely at low fire, reactions are less active and firings are much faster so there is less time for hostile chemistry to affect the color. However underglazes can be made to work well at higher temperatures with more fluid melt transparents and soak-and-rise or drop-and-soak firing schedules.

These two glazes have the same chemistry but different recipes. If it fires too hot like this, then program to fire to cone 5 with a longer soak, or cone 5. Or, program all the steps yourself; that is definitely our preference. Beautiful glazes like this, especially rutile blues, often have serious issues like blistering , crazing , but they can be fixed.

First, the layer is very thick. Second, the body was only bisque fired to cone 06 and it is a raw brown burning stoneware with lots of coarser particles that generate gases as they are heated. Third, the glaze contains zircopax , it stiffens the melt and makes it less able to heal disruptions in the surface. Fourth, the glaze is high in B 2 O 3 , so it starts melting early around F and seals the surface so the gases must bubble up through.

Fifth, the firing was soaked at the end rather than dropping the temperature a little first e. An example of how a carbonate can cause blistering.

Carbonates produce gases during decomposition. An extreme example of blistering in a piece fired at cone Is LOI the issue?

No, this glaze has a low LOI. Low bisque? No, it was bisqued at cone Thick glaze layer? Yes, partly. Holding the firing longer at temperature? No, I could hold this all night and the glaze would just percolate the whole time. Slow cooling? Close, but not quite. The secret I found to fix this was to apply the glaze in a thinner layer and drop-and-hold the temperature for 30 minutes at F below cone Doing that increased the viscosity of the glaze melt to the point that it could break the blisters held by surface tension while still being fluid enough to smooth out the surface.

This is also a common problem at low fire on earthenware clay but can also appear on a buff stonewares. Those white spots you see on the beetle also cover the entire glaze surface although not visible. They are sites of gas escaping from particles decomposing in the body.

The spots likely percolate during soaking at top temperate. Some of them, notably on the almost vertical inner walls of this bowl, having not smoothed over during cool down. What can you do? Use the highest possible bisque temperature, even cone 02 make the glaze thixotropic so it will hang on to the denser body, see the link below about this. Adjust the glaze chemistry to melt later after gassing has finished more zinc, less boron.

Apply a thinner glaze layer more thixotropy and lower specific gravity will enable a more even coverage with less thickness. Instead of soaking at temperature, drop degrees and soak there instead gassing is much less and the increasing viscosity of the melt overcomes the surface tension.

Use a body not having any large particles that decompose and gas on firing. Use cones to verify the temperature your electronic controller reports. When electric kilns, especially large ones are tightly packed with heavy ware, the shady or undersides of the pots simply will never reach the temperature of the element side, no matter how long you soak.

In this example, the inside of this clear glazed cone 6 bowl has a flawless surface. The base is pinholed and crawling a little and the surface of one side the shady side , the remnants of healing disruptions in the melt from escaping gases have not smoothed over.

The element side is largely flawless like the inside, however it is not as smooth on the area immediately outside the foot because this is less element-facing. Industrial gas kilns have draft and subject ware to heat-work by convection, so all sides are much more evenly matured. The buff stoneware mug on the right was bisque fired at cone 02, the one on the left at cone The cone 02 mug was immersed in the clear glaze for 1 second and allowed to dry.

The other was glazed on the inside first, allowed to dry, then glazed on the outside with a 1 second dip. Of course, the cone 02 one took longer to dry. In spite of this, the glaze is thicker and more even on the one bisque fired to cone How is the possible? The secret is the thixotropy of the glaze. When that is right, a one second dip will give the same thickness and evenness whether dry or bisque, 06 or Why bisque fire to cone 02?

To get a glazed surface free of pinholes on some stoneware clays. The clay is terra cotta. Crazing and Peeling. For glossy glaze production, the glaze glossiness on the entire tile surface can be too low. This can be due to low glaze fusibility which can be increased by milling the glaze slightly finer. If this is not sufficient some kaolins listed below are able to increase gloss and give a better glaze finish. This can occur in semi-opaque or opaque glazes due to low crystallization of zircon precipitates during firing.

A high alumina kaolin can improve glaze opacity. Bubbles are always present during and after firing in a glaze. However if the bubbles are too large and are not cured during firing, they will appear as a defect on the glaze surface.

Applying a suitable engobe layer thickness will help to reduce the size of the bubbles which travel through the glaze layer. Then adjusting the molten glaze viscosity can help in one of the two following ways:.

The above adjustment can be achieved in various ways including using a high alumina kaolin which will increase the viscosity of the glaze. Another way to solve this issue is to replace Mg and Ca carbonates by silicates in the body and glaze to reduce the amount of carbon dioxide emitted during firing. This minimizes entrained air and thus imperfections. Certain application techniques produce a better laydown, others produce a fluffier layer e.

If this is your case thinking about ways to densify the dry layer. It is surprising how high the temperature can get and yet steam still be present inside the kiln. All clays release gases from burning of carbon material and decomposition of other compounds. Some clays release sulphur compounds also. If the glaze is melting during release of these gases, they must bubble up through it.

If the melt is stiff, the kiln is ramped up too quickly, cooled too rapidly, or the glaze melts too early, it will not have opportunity to heal properly. Tightly packed electric kilns lacking a venting system require extremely slow and thorough firing especially through the red heat to C range. The superior ventilation in gas kilns makes them best for bisque firing.

A hot bisque is necessary to burn out any sulfur that might be present. A hotter bisque means denser ware and it may be necessary to adjust glazes to be thixotropic so they will apply well to the less absorbent body. Although you may not be accustomed to glazes that will stick to less absorbent bodies, be assured that this is very feasible.

One caution however: If you ware is burnished, it is not usually advisable to bisque above cone 08 or the burnish can be lost. Slow fire through the period where the most gases are generated from the oxidation of organics in the body usually from C to C. This will help you determine the range at which it is most critical to fire slower.

Make sure that reduction does not occur during any phase of bisque or reduced iron FeO could play havoc with latter stage of the firing. This will assure that ware is completely dry and that firing can proceed quickly to past red heat, leaving more time for the carbon burnout phase.

Do blisters get worse even if you fire ware again? This often happens and it is not easy to understand since one would think that there can be no source of gases if the piece has already been glost fired.

Regardless of the reason if a glaze is not healing its blisters on multiple firings then it is not fluid enough. One does not fully appreciate how stiff the average glaze melt is until you work with crystalline glazes that are so fluid a bowl must be placed under the ware to catch the runoff. However the fired surfaces of these glazes are incredibly glossy and perfect.

If your glaze melted more it would run more, however you can counter this by putting it on thinner. The melt fluidity of a glaze is primarily affected by the amount of flux, so you need to increase it. However if the flux you choose has a higher thermal expansion be prepared for the glaze to craze. Fired Pottery Glaze Blisters Problems.

Throwing a Pot Basics. Trimming a Pot Basics.